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Magnetic Fields

The area of influence of a magnet is called a magnetic field . Earth itself has a vast magnetic field emanating from its core and extending far out into space . The fields around much smaller , more familiar magnetized objects follow similar " lines of force " flowing from opposite poles .




Although magnetic fields are invisible , they can be visualizing using tiny pieces of iron called filings . When iron filings are sprinkled around a magnet , they immediately arrange themselves along the field's " lines of force " illustrating the magnetic field .

Every magnet has two ends , or poles : one that is north - seeking ( N ) , and one that is south - seeking ( S ) . If you put together two magnets with like poles facing each other ( N - N or S - S ) , they push each other apart , while if you hold the unlike poles facing each other ( N - S ) , they pull together . Materials that can be magnetized , such as iron , are composed of tiny regions called domains , each of which also has a north and south pole . When iron is unmagnetized , the domains are randomly oriented within the metal . But when the metal is magnetized , the domains all align - forming dense lines with all north - seeking ends pointing in the same direction . If a metal paper clip is held near to a magnet , for instance , it becomes magnetized - its domains align , and it is pulled towards the magnet .


Earth's magnetism


Any magnet that is free to move and carefully balanced on a frictionless bearing will swivel so that the north seeking end always points northwards and the south - seeking end always points southwards ( a compass needle is just a small magnet on a pivot ) . This is because Earth itself has a magnetic field , caused by the powerful electric currents that run through the liquid metal of its outer core . This magnetic field - the magnetosphere - extends into space , where it protects Earth from a stream of harmful particles emitted by the Sun called the solar wind .


Shifting magnetic fields


Although compasses point northwards , they do not point to Earth's geographical north pole , defined by the planet's axis of spin . Compass needles actually point towards " magnetic north " , which at present lies a few hundred kilometers away from the geographical north pole . Earth's magnetic north pole gradually shifts position , as the planet's liquid outer core is free to swirl somewhat separately from the rest of the planet's spin . Occasionally , Earth's magnetic field flips - possibly as a result of turbulence in the liquid metals - so that the magnetic north ends up near the geographical south pole ( in Antarctica ) . This " field reversal " occurs at irregular intervals , from a few hundreds of thousands to many millions of years .


Discovering magnetism


Magnetism was first recorded as a phenomenon around 600 BCB , when people realized that some minerals had magnetic properties . Naturally occurring forms of magnetite , known as lodestones ( meaning " leading stones " ) , were observed always to align themselves in a north - south direction , and were used in the earliest compasses , from around 1000 CE onwards . The first person to study magnetism in a systematic way was English philosopher William Gilbert . In 1600 , he published the results of his investigations , and suggested that our planet acts as a magnet . He also investigated static electricity ( see pp.158-59 ) . Like everyone else at the time , he considered electricity and magnetism to be completely separate . But in 1820 , Danish natural philosopher Hans Christian Ørsted discovered that electric currents produce magnetic fields when the needle of a compass is deflected by a current going through a nearby wire . Physicists quickly came to realize that magnetism and electricity were very closely interrelated , and not separate .



Iron filings around a magnetic field

A magnetic field can be made visible by placing iron filings around a magnet . Each tiny piece of iron lines up with the field , along the " lines of force " . The closer each line is to the next , the stronger the force of the field .


Magnetic forces


A magnet always attracts unmagnetized objects made of iron or steel . But two magnets close together can either attract or repel , depending on whether like or non - like poles are placed nearer each other .





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